[FIX] mail: fix image width inside the chatter

Web editor hard code the width inside the image html. This `max-width`
constraint the image inside the chatter and avoid unwanted horizontal scrollbar.

task-2389432

closes odoo/odoo#62073

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

addons/mail/static/src/components/message/message.scss

@@ -56,6 +56,11 @@
     > p:last-of-type {
         margin-bottom: 0;
     }
+
+    img {
+        max-width: 100%;
+        height: auto;
+    }
 }
 
 .o_Message_core {